Gravure printing press with endless printing belts

ABSTRACT

A printing press includes a large drum driven rotatably. A continuous printing web, of paper or other material, is wound about the circumferential surface of the drum and pulled by the drum from a supply reel and delivered to a driven take-up reel. A plurality of printing units are positioned about the circumferential surface of the rotatable drum at spaced-apart locations. One or more of these units may be gravure units, as for printing different colors. These gravure units include endless gravure belts driven at a speed corresponding to that of the peripheral surface of the drum. The length of the gravure belt is variable, thereby to provide a variable cut off or image length. Floating pressure rollers press the gravure belts against the printing web and against the drum periphery. Alignment means are provided for aligning the gravure belts with the drum.

1111 3,835,775 1451 Sept. 17,1974

H T I W S mm L E B G G N Tl T N RI R P E L Mm GE M 5 Italy 1/1970 Switzerland.........................

[76] Inventor: Robert C. Benton, l8 Canoebrook Primary Examiner j R'eed Fisher Dr., Prmceton Junct1on, NJ. 08550 Attorney Agent or Firm Paul & Paul Oct. 10, 1972 ABSTRACT 22] Filed:

211 App]. No.: 296,003

A printing press includes a large drum driven rotatably. A continuous printing web, of paper or other material, is wound about the circumferential surface of the drum and pulled by the drum from a supply reel [51] Int.

152, 171, -132 2 2 216 and delivered to a dr1ven take-up reel. A plurality of printing units are positioned about the circumferential [58] Field of Search surface of the rotatable drum at spaced-apart locations. One or more of these units may be gravure [56] References Cited UNITED STATES PATENTS units, as for printing different colors. These gravure ,101/179 units include endless gravure belts driven at a speed :9 corresponding to that of the peripheral surface of the Oi/171 drum. The length of the gravure belt 15 var1able, 101/178 thereby to provlde a var1able cut off or 1mage length. 10 Floating pressure rollers press the gravure belts against the printing web and against the drum periphery. Alignment means are provided for aligning the gravure belts with the drum.

1 178 UX 308/9 X 308/9 FOREIGN PATENTS OR APPLICATIONS 1,577,451 4/1969 101/152 14 Claims, 14 Drawing Figures saas'ms PAIENIED SEP! 7 m4 SHEEI 1 OF 6 PATENTED 974 SHEET 2 0F 6 Fin? 3'.aas.115

PAIENIEnsm mu I I I SHEET 3 OF 6 PAIENHDSEP 1 11 1 3. 335315 sum 5 or e PAIENTEDSEPI 11914 SHEET 6 (IF 6 BACKGROUND OF THE INVENTION The present invention relates to printing, and particularly to printing presses. There are presently known a wide variety of methods of printing. These known methods include, for example, relief printing (which includes letterpress and flexography), planographic printing (which includes offset lithography and screenless lithography), gravure printing (which includes photogravure and rotogravure), screen printing (sometimes referred to as silk screen processing), and electrostatic printing (which includes Xerography).

The present invention relates to an improved method and apparatus, primarily for color gravure printing, but relief printing or offset printing units may also be included.

As is known, gravure printing involves the cutting, usually by etching, of various sizes of minute cells or wells below the surface of the plate or cylinder. Ink is applied to the etched surface to flood the wells, after which the surface of the plate or cylinder is scraped of excess ink, as by a doctor blade. The ink remaining in the etched wells is transferred, by a combination of pressure and suction, to a printing web. The character istic feature, in gravure printing, is that the image areas are below the surface of the plate or cylinder. This is in contrast to relief or planographic printing, where the image areas are either on the surface or raised above the surface of the plate or cylinder.

Gravure printing is used advantageously in color printing since the depth of the image cells or wells may be varied, whereby an ink film of varying thickness may be transferred to the printing web. It is the ability of the deeper cells to transfer more ink to the printing web that accounts for the range of tonal values, and the depth of tone, that are characteristic of gravure printmg.

SUMMARY OF THE INVENTION A principal object of the present invention is to provide an improved method and printing press for gravure printing, either in black and white or in a plurality of different colors.

Another object is to provide improved gravure printing means having important economical advantages over techniques heretofore employed.

Another object is to provide, in gravure printing apparatus, means providing a variable cutoff or image length.

Another object is to provide a multi-color gravure printing press capable of printing both sides of the web in a single pass through the press.

Another object is to provide an improved method and apparatus for printing, by gravure and/or other techniques, on one or both sides of a printing web while the plane of the web is vertical.

Another object is to provide, in apparatus as aforesaid, an improved pressure-applying device for holding the etched gravure surface against the printing web.

The foregoing and other objects and advantages are achieved by providing endless gravure belts the lengths of which are variable. The gravure belts are used cooperatively with a large driven drum which functions as the common impression cylinder.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic plan view of a printing press apparatus embodying the present invention, in one form;

FIG. 2 is an elevational view, partly in section, looking along the line 2-2 of FIG. 1;

FIG. 3 is an elevational view looking along the line 3-3 of FIG. 1;

FIG. 4 is an enlarged view of a fragmentary portion of the gravure belt located at the dot-and-dash rectangular area 4 in FIG. 3;

FIG. 3 is a plan view, in section, looking down along the line 5-5 of FIG. 3;

FIG. 6 is an elevational view, in section, looking along the line 6-6 of FIG. 5; I

FIG. 7 is an enlarged view, in section, of the area shown in the dot-and-dash circle 7 in FIG. 6;

FIG. 8 is an elevational view, in section, looking along the line 8-8 of FIG. 1;

FIG. 9 is an enlarged schematic view of the arrangement of the rollers at the relief printing station;

FIG. 10 is an elevational view of the paper supply and take-up reels looking along the line 10-10 of FIG. 1;

FIG. 11 is a view similar to FIG. 1 showing schematically the arrangement for multiple color printing;

FIG. 12 is a view similar to FIG. 1 showing schematically an alternate arrangement for multiple color printmg;

FIG. 13 is a view similar to FIG. 1 showing schematically another alternate arrangement for multiple color printing; and

FIG. 14 is a schematic view in perspective of a dual printing press in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIGS. 1 and 10, a supply roll 20 of continuous printing web W (which will usually be paper but which may be other material) is supported on a reel 21 supported on a vertical shaft 22 for free rotation. By suitable means, as by the piston 23 (FIG. 10), the elevation or vertical position of the roll 20 of web W is adjustable.

After being pulled by a large drum 30 about its circumferential periphery, the web W is wound up on a variable speed take-up reel 91 driven by a motor 92 (FIG. 10). The elevation or vertical position of take-up reel 91 is adjustable, as by a pair of pistons 93 and 94.

Returning now to FIG. I the printing web W travels from the supply roll 20 and, guided by guide rolls 24 and 95, is pulled about the circumferential periphery of a large drum 30 which is mounted for rotation, in the illustrated embodiment, in a horizontal plane about a certical center axis. In the illustrated embodiment, the plane of the continuous web W is vertical as it moves from the supply reel 20 and passes around the vertical periphery of the drum 30. The drum 30 functions as the common impression cylinder for the multi-station apparatus shown in FIG. 1.

A presently preferred form of construction of the common impression cylinder or drum 30 will now be described, with particular reference to FIGS. 1, 2 and 5. Drum 30 is large in diameter. For example, it may have a diameter of the order of 21 feet, and it may be driven at a rotational speed of the order of 60 rpm. The drum 30 is preferably made of ferro concrete having substantially zero contraction. The ferro concrete 31 may be reinforced by radial steel rods 32 which extend from a steel hub 33 radially outwardly to an annular steel band 34 which defines the circumferential periphery of the drum 30.

Drum 30 may preferably be supported for rotation on an air bearing. In FIG. 2, the air bearing is illustrated as a generally circular concrete base having therein an air duct system fed by a high pressure air supply duct 16. The air duct system includes a plurality of radial pipes 116 each having a plurality of vertical jets 37 extending upwardly therefrom which discharge high pressure air toward the under-surface of the drum 30. The air duct system may also include an annular pipe 39 having discharge jets 139 projecting outwardly and upwardly therefrom at inclined angles for projecting air (at lesser pressure than that projected from jets 37) against the inclined surface of the depending annular flange portion 38 of drum 30. By the air means just described, drum 30 is raised sufficiently above the base to allow the driven drum to rotate without frictional engagement between the drum and its base. .lets 139, by projecting air against the inclined surface of the depending annular flange portion 38 assist in guiding the drum in its rotational movement.

Drum 30 may preferably be driven by a hydraulic motor 40 having a rotatable housing 41 mounted on and secured to hub 33 of the drum. The motor housing 41 is driven rotationally about a fixed shaft 42 supported in a fixed plate 44 which in turn may be supported on radially-extending I-beam frame members 45 which are supported, at their outward ends, on vertical columns 46, seen in phantom in FIG. 1. Motor 40 is driven by hydraulic fluid supplied to the motor through lines 43.

As already mentioned, drum 30 may preferably be formed of zero-contraction ferro concrete 3], reinforced by radial steel rods 32, and embraced circumferentially by an annular steel band 34. When drum 30 is rotated at its expected speed, of the order of 60 r.p.m., the centrifugal forces on the steel encircling band 34 are very large. To assist or to enable the band 34 to successfully oppose such centrifugal forces, the steel band 34 may be reinforced with continuous fiberglass filaments 47 wrapped around the outer surface of the steel band 34 and held in place by an epoxy. Alternatively, high tensile steel wire may be wrapped about the steel band in lieu of the fiberglass continuous filaments. A polyurethane film 48 may be sprayed on to the reinforcing filaments (or wire) and cured in place to provide a smooth resilient surface for the printing operation.

Projecting radially from the circumferential surface of drum 30, at regular intervals, are a plurality of locating pins 51 and 52. These pins preferably are rectangular in cross-section, with pins 51 being so oriented that their long dimension is horizontal, while pins 52 are so oriented that their long dimension is vertical. The pins 51 and 52 occupy alternating positions.

Referring again to FIG. 1, and in accordance with the present invention, a plurality of printing units may be positioned about the circumferential surface of drum 30, to function as different printing stations. In FIG. 1, four printing stations are shown. Three of the stations,

identified as 60A, 60B and 60C, are provided with gravure printing units. The fourth station is provided with a relief printing unit.

FIGS. 11, 12 and 13 illustrate schematically other combinations of printing stations positioned about the periphery of drum 30. In FIG. 11, two gravure printing units are indicated. In FIG. 12, two gravure printing units and one relief printing unit are illustrated. In FIG. 13, three gravure printing units and one relief printing unit are shown, with one of the gravure units 60A being extended and having three pressure rolls in sequence, as will be described later. It will be understood that FIGS. 11-13 are merely representative of some of the various combinations of printing stations which may be positioned about drum 30.

In FIG. 1, except for the image information etched into the gravure belts 61, and possibly also except for the color of the ink, printing units 60A, 60B and 60C are identical to each other, each being a gravure print ing unit. Accordingly, it will be necessary to describe but one of these units. Printing unit 60A will be described.

The gravure printing unit 60A includes an endless gravure belt 61 trained about a pair of spaced-apart cylinders 62 and 63 rotatable about vertical axes. In FIG. 1, the cylinders 62 and 63 are so positioned that one reach of the gravure belt 61 is tangent at one point to the peripheral surface of the drum 30. This is the printing point. Gravure belt 61 may be of stainless steel, or ofa copper-aluminum-phosphor alloy, into the surface of which tiny image cells or wells 161 are etched. These wells 161 face, of course, toward the web W and the drum 30.

As seen best in FIG. 3, gravure belt 61 is driven by one cylinder 62 driven as by a hydraulic motor 64. The drive cylinder 62 is fixed in a fixed U-frame 65 and rotates about a vertical axis. The other cylinder 63, which is mounted for free rotation about a vertical axle, is supported in an A-frame 66 the position of which is movable and also adjustable, as by means of a piston 67. Piston 67 may be used to adjust the tension on gravure belt 61, and also, within limits, to change the position of the cylinder 63 to accommodate to gravure belts of different lengths. It is to be noted that one of the important aspects of the printing press of the present invention is the employment of gravure belts, rather than gravure cylinders or plates. The use of gravure belts has the important advantage of accommodating to variable cut-off or variable image lengths by using belts of different lengths. To achieve this objective, it is contemplated by the A-frame 66 may be physically moved, along with piston 67, to a new location to take care of a change in belt length beyond the limits of piston 67.

Both the drive cylinder 62, and also the freely rotatable cylinder 63, are provided with sprocket pins 68 which project radially and which are adapted to be received within sprocket holes 69 provided in the gravure belt 61 along both the upper and lower edge portions. Sprocket holes 69 are seen in FIG. 4. The gravure belt 61 is also provided with a series of generally rectangular holes 71 and 72 for receiving the rectangular locating pins 51 and 52 which project radially from the drum 30. These holes 71 and 72 cooperate with pins 51 and 52 to locate the gravure belts 61, both vertically and laterally, with the peripheral edge of the drum 30.

Ink is applied to the etched surface of the gravure belt by flooding, spraying or other suitable applicator means 73, and the excess ink is scraped from the surface of the belt 61, as by a doctor blade 74 having a back-up roller 75.

As the gravure belt 61 moves, in the direction indicated by the arrow in FIG. 1, beyond the inking means 73 and doctor blade 74, the belt 61 approaches, on a line tangent to the drum 30, the print point located at the point of tangency. At the print point, a floating pressure roller 76 is provided for maintaining the gravure belt 61 pressed against the printing web W which in turn is thereby pressed against the peripheral surface of drum 30. It is largely because of the pressure applied by the pressure roller 76 that the ink in the tiny etched image wells 161 of the gravure belt 61 is transferred to the printing web W.

In accordance with the present invention, and as seen best in FIG. 5, pressure roller 76 is preferably mounted in what may be termed a floating manner. Pressure roller 76 is contained within a chamber 77 of a subhousing 78 mounted on the base of the gravure unit 60A. The walls of the chamber 77, at that half of the sub-housing 78 nearer to the drum 30, are semicircular in cross-section, having a curvature corresponding to the curvature of one half of the peripheral surface of the pressure roller 76. There is, however, a vertical gap or opening 79 in the semi-circular wall of subhousing 78 through which a portion of pressure roller 76 may project.

Air pressure applied to chamber 77 through a conduit 80 urges pressure roller 76 toward the opening 79 in the sub-housing 78 wall and causes a portion of the pressure roller 76 to project beyond the face of the subhousing 78. High pressure air, projected from jets which communicate with high pressure ducts 83, 84, 85 and 86, prevents the wall surface of pressure roller 76 from actually contacting the semi-circular wall of the chamber 77 and keeps the pressure roller 76 centered in the semi-circular portion of the chamber. By the pressurized air delivered through duct 80, the pressure roller 76 is forced outwardly from the interior of chamber 77 and presses against the gravure belt 61, which in turn presses against the printing web W, which in turn presses against the polyurethane surface 48 of the drum 30. When the reaction force of the drum 30, web W, and gravure band 61 equals the applied force of the pressure roller 76, the position of the roller 76 becomes stabilized.

It is to be noted that the printing web W is so positioned vertically, relative to the peripheral surface of drum 30, that the upper edge of web W is below the locating pins 51 and 52 which project horizontally from the drum periphery. These pins 51 and 52 pass through the holes 71 and 72 provided in the gravure belt 61 and extend in an annular recess 176 provided in the peripheral surface of the pressure roller 76. As the gravure belt 61 is pulled past the printing point, the movement of the belt causes the pressure cylinder 76 to rotate about its own axis, in the direction indicated by the arrow in FIG. 5.

Separate motor drives have been shown for the drum 30, printing web W, and gravure belt 61. The speeds at which these drives are operated are, of course, interrelated. The surface speed of gravure belt 61 is slightly higher than the peripheral surface speed of drum 30. Thus, the edges of the locating or alignment holes 71,

72 in the belt pull slightly on the locating pins 51 and 52.

As already indicated hereinabove, and as illustrated inFIG. 1, two additional gravure printing stations 60B and 60C may be provided at spaced-apart locations about the periphery of the drum 30. The equipment at stations 60B and 60C may be identical, or substantially identical, to that at the gravure station 60A. Stations 60B and 60C may, of course, print different information and in different colors. In the equipment illustrated in FIGS. 11 and 12, two different colors may be printed by gravure printing techniques. In the equipment of FIG. 13, three different colors may be gravure printed.

In FIG. 1, and also in FIGS. 12 and 13, a station identified as is illustrated. This is a relief printing station using band techniques. In station 100, a relief band 101 is trained about a driven roller 102 and a freelyrotatable roller 103. The axis of rotation of rollers 102 and 103 is vertical. The freely-rotatable roller 103 is movable and also adjustable, as to the position of its axis, to accommodate to different lengths (variable cutoffs or image lengths). Minor adjustments are made by piston 104. Printing is achieved by means of an inking roll assembly 105. An enlarged view of roll assembly 105 is shown in FIG. 9.

Referring now to FIG. 9, ink is supplied to the nip of a pair of rolls 106 and 107 which are mounted on vertical axes. The outermost roll 106 functions as a doctor roll. The innermost of the two rolls 106 and 107 oscillates back and forth along its axis and may be referred to as the oscillator roll. Oscillator roll 107 contacts the surfaces of two further rolls 108 and 109 which function as spreader rolls. Spreader rolls 108 and 109 are likewise caused to oscillate back and forth along their axes. Spreader rolls 108 and 109 are in contact with a contact inking roll 110 which makes contact with the raised surfaces of the relief images of the relief printing band 101. A back-up roll 111 is provided for the contact inking roll 110.

FIG. 14 is a schematic representation of a dual printing press embodying the features of the present invention and adapted to print on both sides of the web W in a single pass through the press. In FIG. 14, two presses P-1 and P-2 are stacked, one above the other. Each of the presses, P-1 and P-2 may be identical to that of FIG. 1, but, for purposes of simplifying the drawing, the presses P-1 and P-2 have been illustrated as having but two gravure printing units and one relief printing unit. In FIG. 14, the web W is pulled, from the supply roll 20, past vertical guide roller 24-1 and about the periphery of the lower drum 30-1, then out past vertical guide roller 95-1, up past horizontal guide rollers 112-1 and 112-2, past vertical guide roller 24-2, about the periphery of the upper drum 30-2, and out past vertical guide roller 95-2 to the take-up reel 91. In passing from the lower vertical guide roller 95-1 to the upper vertical guide rollers 24-2, the web W, as it passes over the horizontal rollers 112-1 and 112-2, is reversed, so that the side of the web W which was printed by the lower press P-l is now facing the peripheral surface of the upper drum 30-2, thus exposing the other side of the web W to the printing units of the upper press. Thus, by the means illustrated in FIG. 14, both sides of the web W may be printed in a single pass through the pass.

In FIG. 1, a drier 115 is shown following the last of the printing stations for drying the ink on the web W prior to roll-up of the web into roll 90 on take-up reel 91. In some cases, it may be desirable to provide a drier between each printing station, as well as between the final printing station and the take-up reel.

In FIG. 13, gravure printing station 160A illustrates a modification of gravure station 60A described hereinabove. In station 160A, gravure belt 61 is considerably lengthened, and drive roller 62 and free roller 63 are so positioned that belt 61 follows the curved peripheral surface of drum 30, rather than being tangent thereto at one point. A plurality (three) printing points are illustrated, each provided with its own floating pressure roller, each roller housed in its own sub-housing, identified 78-1, 78-2, 78-3. By the means illustrated, the printing image on the web W becomes more dense at each of the printing stations.

Other variations and modifications may be provided within the concept disclosed.

For example, while drum 30 has been described as preferably supported on an air bearing, it may, of course, be otherwise supported. Or, in lieu of an air bearing, an oil bearing may be used.

In the preferred embodiments, the axis of the drum 30 is vertical, the printing web W is vertical, and the image-carrying surface of the gravure belt 61 is vertical. In these embodiments, ink is retained in the tiny image cells 116 by capillary action. It is contemplated, however, that endless gravure belts 61 may also be used in presses in which the drum 30 is rotated on a horizontal axis, and in which the web W is printed by gravure belts which are horizontally disposed.

What is claimed is:

1. Printing apparatus comprising:

a. a cylindrical drum mounted for rotation;

b. a continuous printing web entrained about the peripheral surface of said drum;

c. at least one movable endless belt carrying image areas on a surface facing the web on the peripheral surface of said drum and adjacent thereto at at least one point;

d. means for rotating said drum;

e. means for moving said image-carrying belt;

f. pressure means for urging said image-carrying belt against said web on the peripheral surface of said drum at said point of adjacency;

g. said pressure means comprising a floating pressure roller;

h. a sub-housing having a chamber in which said pressure roller is floated;

. said sub-housing having a wall facing said web, said wall having an opening therein; and

j. pressurized air supply means providing high pressure air to said chamber for urging said floating pressure roller toward said opening in said wall of said sub-housing and for causing a portion of said pressure roller to project from said wall of said subhousing.

2. Printing apparatus according to claim 1 wherein:

a. said movable image-carrying belt is an endless gravure belt having image wells therein. 3. Printing apparatus according to claim I wherein:

a. a pair of spaced-apart sprocket rollers are provided each rotatable about a vertical axis and about which said endless belt is trained;

b. said sprocket rollers are so positioned that one reach of said belt is along a line tangent to the periphery of said drum at said point of adjacency.

4. Printing apparatus according to claim 1 wherein a plurality of movable image-carrying endless belts are provided at spaced-apart positions about the periphery of said drum.

5. Printing apparatus according to claim 4 wherein:

a. at least two of said movable image-carrying belts are gravure belts each of which has a reach movable along a line tangent to the periphery of said drum at a different spaced-apart point;

b. one of said gravure belts carries in its image wells ink of one color and the other of said gravure belts carries in its image wells ink of another color.

6. Printing apparatus according to claim 1 wherein:

a. said cylindrical drum is made of ferro concrete with steel rod reinforcements;

b. said concrete has an annular steel band embracing its periphery;

c. said annular steel band is reinforced against centrifugal forces by having wrapped thereabout a continuous filament of high-strength material.

7. Printing apparatus according to claim 6 wherein said high-strength material is mono-filament fiberglass.

8. Printing apparatus according to claim 6 wherein said high-strength material is high tensile wire.

9. Printing apparatus according to claim 6 wherein:

a. the peripheral surface of said drum is provided with radially-extending locating pins;

b. the peripheral surface of said floating pressure roll is provided with an annular recess for receiving the projecting ends of said locating pins.

10. Printing apparatus according to claim 1 wherein:

a. high pressure fluid means are provided for lifting said drum slightly off its mounting to allow for substantially friction-free rotation of said drum.

11. Printing apparatus according to claim 1 wherein:

a. said continuous web is obtained from a supply roll and wound around the peripheral surface of said drum;

b. take-up means are provided for receiving said web after it has passed around the periphery of said drum;

c. drive means are provided for driving said take-up means.

12. Printing apparatus according to claim 1 wherein:

a. said drum is mounted for rotation in a horizontal plane about a vertical axis;

b. said continuous printing web is vertically disposed on the vertical peripheral surface of said drum;

c. said image area surface of said endless belt is vertically disposed at the point of adjacency to said web.

13. Printing apparatus according to claim 12 wherein:

d. pressure means are provided for urging said second image-carrying belt against said interposed web andsaid peripheral surface of said' second drum; and

e. said continuous web being common to said first and second drums but having opposite surfaces facing said first and second image-carrying belts.

14. Printing apparatus according to claim 1 wherein:

a. said wall of said sub-housing facing said web having an opening therein toward which said floating pressure roller is urged by said pressurized air supply means is semi-circular in cross-section;

b. said pressurized air supply means includes air jets positioned to prevent said roller from engaging the surface of said semi-circular wall. 

1. Printing apparatus comprising: a. a cylindrical drum mounted for rotation; b. a continuous printing web entrained about the peripheral surface of said drum; c. at least one movable endless belt carrying image areas on a surface facing the web on the peripheral surface of said drum and adjacent thereto at at least one point; d. means for rotating said drum; e. means for moving said image-carrying belt; f. pressure means for urging said image-carrying belt against said web on the peripheral surface of said drum at said point Of adjacency; g. said pressure means comprising a floating pressure roller; h. a sub-housing having a chamber in which said pressure roller is floated; i. said sub-housing having a wall facing said web, said wall having an opening therein; and j. pressurized air supply means providing high pressure air to said chamber for urging said floating pressure roller toward said opening in said wall of said sub-housing and for causing a portion of said pressure roller to project from said wall of said sub-housing.
 2. Printing apparatus according to claim 1 wherein: a. said movable image-carrying belt is an endless gravure belt having image wells therein.
 3. Printing apparatus according to claim 1 wherein: a. a pair of spaced-apart sprocket rollers are provided each rotatable about a vertical axis and about which said endless belt is trained; b. said sprocket rollers are so positioned that one reach of said belt is along a line tangent to the periphery of said drum at said point of adjacency.
 4. Printing apparatus according to claim 1 wherein a plurality of movable image-carrying endless belts are provided at spaced-apart positions about the periphery of said drum.
 5. Printing apparatus according to claim 4 wherein: a. at least two of said movable image-carrying belts are gravure belts each of which has a reach movable along a line tangent to the periphery of said drum at a different spaced-apart point; b. one of said gravure belts carries in its image wells ink of one color and the other of said gravure belts carries in its image wells ink of another color.
 6. Printing apparatus according to claim 1 wherein: a. said cylindrical drum is made of ferro concrete with steel rod reinforcements; b. said concrete has an annular steel band embracing its periphery; c. said annular steel band is reinforced against centrifugal forces by having wrapped thereabout a continuous filament of high-strength material.
 7. Printing apparatus according to claim 6 wherein said high-strength material is mono-filament fiberglass.
 8. Printing apparatus according to claim 6 wherein said high-strength material is high tensile wire.
 9. Printing apparatus according to claim 6 wherein: a. the peripheral surface of said drum is provided with radially-extending locating pins; b. the peripheral surface of said floating pressure roll is provided with an annular recess for receiving the projecting ends of said locating pins.
 10. Printing apparatus according to claim 1 wherein: a. high pressure fluid means are provided for lifting said drum slightly off its mounting to allow for substantially friction-free rotation of said drum.
 11. Printing apparatus according to claim 1 wherein: a. said continuous web is obtained from a supply roll and wound around the peripheral surface of said drum; b. take-up means are provided for receiving said web after it has passed around the periphery of said drum; c. drive means are provided for driving said take-up means.
 12. Printing apparatus according to claim 1 wherein: a. said drum is mounted for rotation in a horizontal plane about a vertical axis; b. said continuous printing web is vertically disposed on the vertical peripheral surface of said drum; c. said image area surface of said endless belt is vertically disposed at the point of adjacency to said web.
 13. Printing apparatus according to claim 12 wherein: a. a second cylindrical drum is provided mounted for rotation about a vertical axis and having a substantially vertical peripheral surface; b. at least one movable second endless belt which carried image areas on a substantially vertical surface is provided for said second drum, said second image-carrying surface facing the peripheral surface of said second drum and being adjacent thereto at at least one point; c. means are provided for interposing said continuous web in vertically-disposed positioN between said second image-carrying belt and said peripheral surface of said second drum at said point of adjacency; d. pressure means are provided for urging said second image-carrying belt against said interposed web and said peripheral surface of said second drum; and e. said continuous web being common to said first and second drums but having opposite surfaces facing said first and second image-carrying belts.
 14. Printing apparatus according to claim 1 wherein: a. said wall of said sub-housing facing said web having an opening therein toward which said floating pressure roller is urged by said pressurized air supply means is semi-circular in cross-section; b. said pressurized air supply means includes air jets positioned to prevent said roller from engaging the surface of said semi-circular wall. 